A wheel and a braking rotary member of a motor vehicle are supported by a rolling bearing unit so as to be rotatable with respect to a suspension system. During the cornering of the motor vehicle, a wheel supporting rolling bearing unit like this is subjected to a large moment. Therefore, in order to ensure stability during the cornering, a rolling bearing unit having high moment rigidity is required. To this end, wheel supporting rolling bearing units generally adopt a configuration in which rolling elements are arranged in double rows, and in which a preload and contact angles of a back-to-back arranging type are given to the double rows of rolling elements respectively. Further, in recent years, with a view to ensuring higher moment rigidity while keeping a rolling bearing unit from becoming oversized, there is proposed a configuration in which pitch circle diameters or rolling element diameters are made different between the double rows rolling elements (see, e.g., JP 2003-232343 A, JP 2004-108449 A, JP 2004-345439 A, JP 2006-17365 A, and International Publication WO2005/065007).
FIG. 5 shows a wheel supporting rolling bearing unit 1 described in JP 2006-137365 A. This rolling bearing unit 1 includes a hub 2, an outer ring 3, and a plurality of balls (rolling elements) which are arranged in double rows. The hub 2 includes a hub body 5 and an inner ring 6. The hub 2 is formed with a mounting flange 7 on an axially outer end portion of an outer circumferential surface to support a wheel and a braking rotary member, and double rows of inner ring raceways 8a, 8b on an axially intermediate portion and on an axially inner end portion of the outer circumferential surface, respectively. In the following description, being outer with respect to the axial direction means being on an outer side in a widthwise direction of a vehicle when the rolling bearing unit is installed in the vehicle, which is a left side in FIGS. 5 and 6. Being inner with respect to the axial direction means being on a right side in FIGS. 5 and 6, which is a center side in the widthwise direction of the vehicle. A diameter of the inner ring raceway 8a of the axially outer row is larger than a diameter of the inner ring raceway 8b of the axially inner row. For the purpose of supporting and fixing the braking rotary member, such as a disk, or the wheel to the mounting flange 7, base end portions of a plurality of studs 9 are fixed to the mounting flange 7.
In order to make the diameters of the inner ring raceways 8a, 8b different, an outer circumferential sloping stepped portion 10 is formed on an outer circumferential surface of an axially intermediate portion of the hub body 5 at a portion which is slightly inward in the axial direction from the inner ring raceway 8a of the outer row, so as to be sloped in a direction in which its outside diameter decreases as it extends axially inward. On an axially inner end portion of the hub body 5, which is further inward in the axial direction than the outer circumferential sloping stepped portion 10, a small diameter stepped portion 11 is formed. On this small diameter stepped portion 11, an inner ring 6 having an outer circumferential surface on which the inner ring raceway 8b of the inner row is formed is fitted. This inner ring 6 is pressed against a stepped surface 13 at an axially outer end portion of the small diameter stepped portion 11 by a rivet portion 12, which is formed at an axially inner end of the hub body 5, and is joined to the hub body 5 in a fastened manner. The inner ring raceways 8a, 8b have an arcuate cross-sectional shape (a generatrix shape), respectively, such that the respective outside diameters decrease as they become close to each other, i.e., as they extend towards the center of the hub 2 in the axial direction.
The outer ring 3 is formed with double rows of outer ring raceways 14a, 14b on an inner circumferential surface thereof and a coupling flange 15 on an inner circumferential surface thereof for being coupled to the suspension system in a fastened manner. A diameter of the outer ring raceway 14a of the axially outer row is larger than a diameter of the outer ring raceway 14b of the axially inner row. Thus, on an inner circumferential surface of an axially intermediate portion of the outer ring 3 at a portion which is slightly inward in the axial direction from the outer ring raceway 14a of the outer row, an inner circumferential sloping stepped portion 16 is formed so as to be sloped in a direction in which its inside diameter decreases as it extends axially inward. The outer ring raceways 14a, 14b have an arcuate cross-sectional shape (a generatrix shape), respectively, such that the respective inside diameters decrease as they become close to each other, i.e., as they extend towards the center of the hub 2 in the axial direction.
The double rows of a plurality of the balls 4 are rollably arranged between the inner ring raceways 8a, 8b and the outer ring raceways 14a, 14b. In this arrangement, a preload and contact angles of a back-to-back arranging type (a DB type) are given to the double rows of balls 4, respectively. Pitch circle diameters of the balls 4 in the respective rows are different from each other in accordance with a difference in diameters between the inner ring raceways 8a, 8b and a difference in diameters between the outer ring raceways 14a, 14b. That is, a pitch circle diameter PCDout of the balls 4 in the axially outer row is larger than a pitch circle diameter PCDin of the balls 4 in the axially inner row (PCDout>PCDin).
According to the configuration described above, the moment rigidity is increased in accordance with the increase of the pitch circle diameter PCDout of the outer row. Therefore, it becomes easier to design for improving running stability during the cornering and for improving durability of the wheel supporting rolling bearing unit. On the other hand, because the pitch circle diameter PCDin of the inner row does not have to be increased, the running stability and the durability can be improved without specially increasing a diameter of a portion (e.g., a knuckle mounting hole) of the suspension system.
In the configuration described above and shown in FIG. 5, the diameters of the balls 4 arranged in the double rows are the same. On the other hand, as shown in FIG. 6, there is proposed a wheel supporting rolling bearing unit 1a in which a diameter of balls 4a in an outer row is smaller than a diameter of balls 4b in an inner row. In this case, the number of balls 4a in the outer row is made greater than the number of balls 4b in the inner row so as to increase rigidity of the outer row than rigidity of the inner row. Further, while the balls 4 (4a, 4b) are used as rolling elements in the examples of FIGS. 5 and 6, tapered rollers may be used as rolling elements in a rolling bearing unit for heavyweight vehicles.
When manufacturing the outer ring 3 of the rolling bearing units 1, 1a described above, as in the case of an outer ring of a rolling bearing unit in which pitch circle diameters of rolling elements in inner and outer rows are the same, a hot forging process may be carried out on a stock 17 of an iron alloy, such as a medium-carbon steel, in the steps (A) to (F) shown in FIG. 7. That is, an intermediate work is produced by carrying out a plastic forming on the stock 17 so as to have a slightly larger size than an external shape of the outer ring 3, and a shaping, such as a turning, and a finishing, such as a grinding, are carried out on the intermediate work. By carrying out the plastic forming, such as the hot forging, prior to the shaping and the finishing, the metallic structure of the outer ring 3 is densified, whereby the strength of the outer ring 3 is improved. Further, the cutting amount of the material is reduced, whereby cost reduction can be realized as a result of improved yield rate of the material and shortened machining time.
In the manufacturing method described above, firstly, an elongated material is cut into a certain length to form a solid solid cylindrical stock 17 shown in (A) of FIG. 7. Next, an upset process is carried out in which the stock 17 is flattened in an axial direction to expand a diameter thereof, whereby a primary intermediate work 18 having a beer barrel shape as shown in (B) of FIG. 7 is formed.
Subsequently, a rough forming is carried out in which the primary intermediate work 18 is plastically deformed in a mold unit having a punch and a counter punch, whereby a preliminary secondary intermediate work 19 is formed. The preliminary secondary intermediate work 19 has a cylindrical wall 20 and a partitioning portion 21 which closes an inside of the cylindrical wall 20 at an axially intermediate portion. On an outer circumferential surface of the axially intermediate portion of the cylindrical wall 20, a blank flange portion 22, which becomes a coupling flange 15 (see FIGS. 5 and 6), is formed.
Through a finishing forming process in which the preliminary secondary intermediate work 19 is plastically deformed in another mold unit having a different punch and a counter punch, a subsequent preliminary secondary intermediate work 23 as shown in (D) of FIG. 7 is formed. In the step of processing the preliminary secondary intermediate work 19 into the subsequent preliminary secondary intermediate work 23, an overall shape is arranged by reducing thicknesses of the partitioning portion 21 and the blank flange portion 22. On an outer circumferential edge of a secondary blank flange portion 24 of the subsequent preliminary secondary intermediate work 23 obtained by the finish forming, a burr 25 is formed as a result of the residual material. Thus, the burr 25 is removed to obtain a secondary intermediate work 26 as shown in (E) of FIG. 7.
A punching is carried out on the secondary intermediate work 26 to remove the partitioning portion 21, whereby a tertiary intermediate work 27 shown in (F) of FIG. 7 is obtained.
Further, the outer ring raceways 14a, 14b are formed in double rows on inner circumferential surface of the cylindrical wall 20 of the tertiary intermediate work 27 at two locations in the axial direction by turning and grinding, whereby the outer ring 3 is having the double rows of outer ring raceways 14a, 14b and the coupling flange 15 is obtained.
In the case of manufacturing the outer ring 3 in the way described above, flexibility in setting the positions and diameters of double rows of outer ring raceways is low for the following reasons.
The hot forging in (B) to (C) of FIG. 7, which is the plastic forming, is called a forward-backward extrusion, in which the respective end surfaces of the heated primary intermediate work 18 is strongly pressed between the punch and the counter punch which are concentrically disposed inside the die. By this pressing operation, a part of the primary intermediate work 18 is extruded forward in a punch push-in direction (a forward extrusion), and at the same time, the remaining part of the primary intermediate work 18 is extruded rearward in the punch push-in direction (a backward extrusion).
A load (deformation resistance) required for the forward extrusion is larger than a load required for the backward extrusion. That is, in the forward extrusion which is carried out in the process of (B) to (C) in FIG. 7, a metal material squashed between the punch and the counter punch is moved into an annular space between an inner circumferential surface of the die which lies further forward in push-in direction than the punch and an outer circumferential surface of the counter punch. A resistance against the deformation of the primary intermediate work 18, which is made of an iron alloy such as medium-carbon steel, is still quite large in the heating level for the hot forging. Therefore, the position of the partitioning portion 21 in the axial direction is required to be set in a range that enables the forward extrusion. On the other hand, the positions of the pair of outer ring raceways formed on the inner circumferential surface of the completed outer ring 3 are positions which interpose the partitioning portion 21 from respective sides in the axial direction. In other words, an amount to be processed by the forward extrusion is determined depending on the positions of arranging the outer ring raceways.
Under such conditions, it is difficult to manufacture the outer ring 3, in which diameters of the pair of outer ring raceways 14a, 14b are different from each other, at low cost. That is, once the tertiary intermediate work 27 shown in (F) of FIG. 7 is produced, the diameters of the pair of outer ring raceways are almost fixed. That is, the diameters of the outer ring raceways become larger than inside diameters of portions of the tertiary intermediate work 27 where the outer ring raceways are to be formed, by an amount to be cut in the finishing process such as turning and grinding. Further, in order to make a difference in the diameters of the outer ring raceways larger than the difference in the inside diameters of the portions of the tertiary intermediate work 27 where the outer ring raceways are to be formed, a cutting amount in forming one of the outer ring raceways needs to be larger than a cutting amount in forming the other of the outer ring raceways. Making the diameters of the outer ring raceways different in the way described above not only extends the machining time but also deteriorates the yield rate of the material, causing an increase in the manufacturing cost of the outer ring 3. Further, in the manufacturing method described above, the amount to be processed by the forward extrusion necessarily becomes large to some extent, and more time has to be spent in carrying out the process of (B) to (C) in FIG. 7 accordingly. Thus, in this respect also, the machining time is extended, and the manufacturing cost of the outer ring 3 is increased.